Demands placed on wireless communications devices are continually increasing. While the number of functions required of wireless communications devices is increasing, consumers are simultaneously demanding lower cost devices with longer use times between battery charging and which are lighter and smaller than previous devices.
Particularly, there is a high demand for adding global positioning systems (GPS) services to the functions of wireless communications devices. More specifically, consumers want a wireless communications device that has both GPS and PCS services or GPS and cellular CDMA, among other services. Additionally, some government agencies, e.g., the FCC in the United States, are mandating that position location functionality be incorporated into wireless communications devices which is able to meet a specific accuracy. GPS is one solution which can meet this requirement and operates at a frequency of 1575.42 MHz, as is well known in the art. PCS is a wireless communication band at a frequency of about 1960 MHz, as is well known in the art. Cellular CDMA is a wireless communication system at a frequency of about 870 MHz, as is well known in the art.
In a typical wireless communications device receiver, an antenna assembly receives a radio frequency (rf) signal from the air. The antenna assembly may includes an antenna, an antenna matching circuit and any other component required to receive the rf signal. The signal is a series of electromagnetic fields travelling in waves in the air at a certain frequency, the radio frequency. The waves carry energy. The antenna captures some of this energy so that the energy can travel along a guided path. The guided path may be a wave guide or metal conductors, such as wires or strips of metal, known as microstrip lines, on a substrate, among other things.
The signal captured by the antenna may have waves of frequencies other than the desired rf. Filters are used to block any signal component at frequencies other than the desired rf. Thus, the rf signal is made to travel through one or more filters to select a desired frequency or range of frequencies. Also, the desired rf signal may be very weak. Amplifiers are used to magnify the strength of the rf signal. Thus, the rf signal is made to pass through one or more amplifiers.
The rf signal carries information. The information is contained in changing phase, amplitude or frequency of the rf signal. To get this information out of the signal, the signal is typically compared to an rf signal of constant phase, amplitude or frequency. The constant phase, amplitude or frequency rf signal is known as a local oscillator signal and comes from a precise rf signal generator in the communication device, known as a local oscillator. The local oscillator signal is combined with the rf signal by a device known as a mixer. The mixer produces sum and difference signals of the local oscillator signal and the rf signal. This process of producing sum and difference signals from two initial signals is known as mixing, producing mixed signals.
Commonly, the rf signal is mixed once down to an intermediate frequency (IF) signal and then again down to what is known as a baseband signal. Both the IF signal and the baseband signal are lower frequency signals. A baseband signal contains the coded information that the original rf signal was carrying. When the rf signal is mixed down to an IF signal, one of the mixed signals is typically chosen to carry the message contained in the rf signal. This chosen signal is known as the IF signal. The IF signal is typically selected by an IF filter designed to transmit the IF signal only.
After the IF filter, and possible further filtering and amplification, the signal encounters another mixer, where it is mixed with another local oscillator signal. This local oscillator signal has a frequency equal to the IF signal frequency. Again, sum and difference signals are produced. The signal of interest at this point is known as the baseband signal. The baseband signal is also known as the “zero frequency difference” signal, even though the frequency of the baseband signal is not zero. Its frequency is the frequency of the coded information that it carries. The baseband signal is then decoded to reveal whatever information was transmitted. It will be appreciated that the signal may be mixed directly from an rf signal down to baseband. In this case, there is no IF signal.
When GPS service is added to a wireless communication device, the GPS receiver portion typically has each of the above devices to process the GPS rf signal. In various designers' attempts to combine PCS and GPS services in one device, separate mixers have been used for down converting PCS to an intermediate frequency (IF) and for down converting GPS to an IF. Additionally, separate IF filters have been used to filter the IF signal after down conversion to IF and before down conversion to baseband. The same is true of attempts to combine GPS and cellular CDMA. Separate mixers and IF filters have been used. Thus, the cost, size and power consumption of prior art GPS enabled devices is significantly more than that of those without GPS services.